1. Field of the Invention
This invention relates to a transparent conductive thin film that is used as a liquid-crystal display (LCD) element, organic electroluminescence (EL) element or solar cell display.
2. Description of the Related Art
Transparent conductive thin film has a high level of conductivity (for example, specific resistance of 1×10−3Ω·cm or less) and a high transmission factor in the visible light range, so in addition to being used in the terminals of solar batteries, liquid-crystal display elements and other light-receiving elements, it is also used as; heat-reflective film for the window glass of automobiles and buildings; anti-static film; and as the transparent anti-fog heating clement in refrigerated showcases.
Tin oxide (SnO2) film doped with antimony or fluorine, zinc oxide (ZnO) film doped with aluminum or gallium, or indium oxide (In2O3) doped with tin have been widely used for transparent conductive thin film. Particularly, iridium oxide doped with tin, or in other words, In2O3—Sn type film, called ITO (Indium tin oxide), is often used because it can be easily made into a film having low resistance.
A sputtering method is often used as the method for manufacturing these transparent conductive thin films. The sputtering method is an effective method when using a material with low vapor pressure to form a film on an object hereafter called the ‘substrate’), or when the thickness of the film must be precisely controlled, and it is widely used because operation is very simple.
The sputtering method is usually performed under conditions of argon gas at a pressure of 10 Pa or less and using the substrate as the anode and target as the cathode, between which a glow discharge occurs to generate argon plasma such that the positive argon ions in the plasma collide with the cathode target and break off the particles of The target and to deposit those particles on the substrate to form a film.
The sputtering method is classified by the method used to generate argon plasma, so a method that uses high-frequency plasma is called a high-frequency sputtering method, and a method that uses direct-current plasma is called a direct-current sputtering method Also, a method in which a magnet is placed behind the target to form a film by concentrating the argon plasma directly onto the target to improve the collision rate of the argon ions even under low gas pressure is called a magnetron sputter method. Normally, a direct-current magnetron sputter method is used as the method for manufacturing transparent conductive thin film.
A smooth transparent conductive film is necessary for the surface of the electrodes of an LCD or organic EL display Particularly, in the case of the electrodes for an organic EL display, an ultra-thin film made from organic compounds is formed on top, so a very smooth surface is necessary. Generally, surface smoothness is largely influenced by the crystallinity of the film. Even for film with the same composition, an amorphous film with no grain boundaries has very good surface smoothness.
In the case of the prior ITO structure, an amorphous ITO film that is formed by lowering the substrate temperature and performing sputtering at low temperature (150° C. or lower) and high pressure (1 Pa or greater) has excellent surface smoothness. However, the specific resistance of an amorphous ITO film is limited to 6×10−4Ω·cm, and in order to form an electrode film having low surface resistance, the film itself must be made to be thick. When the film thickness of the ITO film is tick, a problem of film coloration occurs.
Also, in the case of an ITO film that is formed at room temperature without heating The substrate, when the sputter gas pressure is low, the kinetic energy of the sputter particles input to the substrate is high, so a localized temperature rise occurs, and a film is formed having a minute crystal phase and amorphous phase. In addition to X-ray diffraction, the existence of a minute crystal phase can be confirmed by using a transmission electron microscope or by electron beam diffraction. When this kind of minute crystal phase is formed in part of the film, the surface smoothness of the film is greatly affected. Moreover, when using a weak acid to perform etching removal to form the prescribed shape of the film, there is a problem in that it is not possible to remove just the crystal phase and some may remain.
Besides the problem with specific resistance, amorphous ITO film has a problem with stability. When an amorphous ITO film is used for the electrodes of an LCD or organic EL display, crystallization of the electrode film occurs when heating is performed at 150° C. or greater by thermal hysteresis during the manufacturing process after the electrode has been formed. This is because the amorphous phase is a metastable phase. When crystallization occurs, crystal grains are formed, and there is a problem in that the surface smoothness becomes poor and specific resistance varies greatly.
Next, the organic electroluminescence element will be explained. An electroluminescence element (hereafter abbreviated as EL element) employs electroluminescence, and since it is self emitting and so high in visibility and since it is a completely solid-state element, it is very impact resistant, therefore its use has gain much attention as a light-emitting element for all kinds of display devices. There are inorganic EL elements that use inorganic compounds as the light-emitting material, and there are organic EL elements that use organic compounds which includes polymertype organic compounds. Of these, it is easy to make organic EL elements that are small in size and have a greatly lowered drive voltage, so research for application in next-generation display elements is actively being pursued. Organic EL elements are basically constructed with an anode, light-emitting layer and cathode, and are normally used for forming a transparent anode on a glass substrate or the like. In this case, light emission is performed on the substrate side.
Recently, for the reasons explained below, attempts have been made to make the anode transparent and to perform light emission from the anode side. First, by making the cathode transparent along with the anode, a totally transparent light-emitting element is possible. Any arbitrary color can be used as the background color of the transparent light-emitting element, which makes it possible to have a colorful display even when light is not emitted and thus improves appearance. Using black as the background color improves contrast during light emission. Next, when using a color filter or color transformation layer, it is possible to place it on top of the light-emitting element. Therefore, it is possible to manufacture the element without having to consider these layers. One advantage of this, for example, is that it is possible to raise the temperature of the substrate when forming the anode, and this makes it possible to lower the resistance of the anode.
Also, the advantage described above can also be obtained by making the cathode transparent, so attempts are being made to manufacture organic EL elements that use transparent cathodes. For example, the organic EL element disclosed in Japanese patent publication Tokukai Hei 10-162959 comprises an organic layer that includes an organic light-emitting layer located between an anode and cathode, and the cathode has a metal layer that is injected with electrons and an amorphous transparent conductive layer such that the electron-injection metal layer comes in contact with the organic layer. Also, in Japanese patent publication Tokukai 2001-43980, an organic EL element is disclosed in which the cathode is made to be transparent and a light-reflecting metal film is used for the anode, and light is effectively obtained from the cathode.
Next, the electron-injection metal layer will be explained. The electron-injection metal layer is a metal layer in which electrons can be injected in an organic layer that includes the light-emitting layer, and in order to obtain a transparent light-emitting element, it is preferable that the light transmittance be 50% or greater, and therefore it is preferable that the film thickness be ultra thin or about 0.5 to 20 nm. A metal (electron-injection metal) such as Mg, Ca, Ba, Sr, Li, Yb, Eu, Y or Sc having a work function of 3.8 cV or less and a thickness of 1 nm to 20 nm can be used as the electron-injection metal layer. In this case, it is preferred that the light transmittance be 50% or greater, and more preferably 60% or greater.
The organic layer between the anode and cathode includes at least a light-emitting layer. The organic layer can also be just a light-emitting layer, or together with the light-emitting layer it is possible to have multi-layer construction with layers such as an electron-hole transport layer. In the organic EL element, the organic layer can (1) have the function of injecting electron holes by the anode or electron-hole transport layer when an electric field is applied and the function of injecting electrons by an electron-injection layer; (2) have a transport function that moves the injected electric charge (electron and electron hole) by the force of the electric field; (3) provide a place inside the light-emitting layer for recombining the electrons and electron holes, and have a light-emitting function. The electron-hole injected transport layer is a layer made from an electron-hole transfer compound and has the function of transferring electron-holes that were injected by the anode to the light-emitting layer, and by placing this electron-hole injected transport layer between the anode and light-emitting layer, many electron holes can be injected into the light-emitting layer with a weaker electric field. In addition, the electrons that are injected into the light-emitting layer by the electron-injection layer are obstructed by an electron barrier that exists at the boundary between the light-emitting layer and the electron-hole transport layer and accumulate near the boundary in the light-emitting layer and improve the light-emitting efficiency of the EL element to make for an EL element with excellent light-emitting capability.
The anode is not particularly limited as long as the work function indicates conductivity of 4.4 eV or greater, more preferably 4.8 eV or greater. A metal, transparent conductive film (conductive oxide film) or a combination of these having a work function of 4.8 eV or greater is preferred. The anode does not necessarily have to be transparent, and can be coated with a black carbon layer or the like. Suitable metals include, for example, Au, Pt, Ni and Pd, and the conductive oxide can be, In—Zn—O, In—Sn—O, ZnO—Al or Zn—Sn—O. Also, combinations could include laminated layers of Au and In—Zn—O, Pt arid In—Zn—O, or In—Sn—O and Pt. Moreover, since the anode is suitable as long as the boundary with the organic layer has a work function of 4.4 eV or greater, the anode can be two layered, and a conductive layer having a work function of 4.4 eV or less can be used on the side that does not come in contact with the organic layer. In this case, a metal such as Al, Ta or W, or an alloy such as Al alloy or Ta—W alloy can be used. Also, it is possible to use a conductive polymer such as doped polyaniline or doped polyphenylenevinylene, or an amorphous semiconductor such as a-Si, a-SiC, or a-C, furthermore, a black semiconductive oxide such as Cr2O3, Pr2O3, NiO, Mn2O5 or MnO2 etc.
It is preferable that the transparent conductive layer of the cathode in the organic EL element be an amorphous transparent conductive layer with low film stress and good smoothness. Also, in order to eliminate voltage drops and the unevenness in light emission that is caused by it, it is preferred that the specific resistance be 6×10−4Ω·cm or less.
Indium oxide film containing tungsten and/or molybdenum is well known. For example, Japanese patent publication Tokuko Sho 50-19125 discloses a technique of manufacturing an indium oxide film containing tungsten or molybdenum on a glass substrate heated to 350° C. by the electron-beam evaporation method. However, the aim of the indium oxide film containing tungsten or molybdenum of this disclosure is an indium oxide film with low resistance, and not that of improving the surface smoothness of the film or raising the crystallization temperature In this disclosure, nothing is mentioned about making an amorphous film or improving the surface smoothness of the film and it is not stated that the crystallization temperature of the film is raised by adding tungsten and/or molybdenum.
An amorphous film is not obtained by simply adding tungsten and/or molybdenum as in the case of the indium oxide film containing tungsten and/or molybdenum described in the disclosure mentioned above, and it is not possible to obtain an amorphous indium oxide film containing tungsten and/or molybdenum when the film is formed at 350° C. as described above. It is not possible to obtain an amorphous film by simply adding tungsten in this way, and neither is it possible to obtain a film with excellent surface smoothness as well as low resistance and high trasmissivity.
Obtaining a transparent conductive film with excellent surface smoothness and that is stable even in the thermal hysteresis of the manufacturing process was impossible using conventional ITO material, and thus it was difficult to use the film in the transparent electrodes of display elements such as an organic EL display or LCD.